Hi Lenny:
- By way of explanation let me first note that most of my experience has been
with military, space, or similar high-reliability product applications.
Within the confines of those hi-rel applications, any gold plating less than
50 microinches was rejected as insufficient.  The plating requirements were
that gold be applied in accordance with Mil-G-45204 over a nickel plating
which had been applied in accordance with QQ-N-290.  Gold was to be from 50 to
100 microinches thick.  Gold thicker than 100 microinches had to be removed,
usually via double tinning.  When a thin "gold flash" (less than 30
microinches) is applied, the deposition speed and thickness usually combine to
produce a porous finish which is relatively transparent to moisture and leads
to oxidation beneath the thin plating.  Studies by the Tin Institute (and
others) confirmed that up to 100 microinches of gold would not result in a
high enough concentration of gold (>3%) in a plated through hole solder
connection for the connection to be brittle.  Note that the studies were done
on PLATED THROUGH HOLES where a significantly greater volume of solder is
present than on a SMT connection.
- Today's surface mount connections with minimum solder quantity and the
present plating processes with ultra-thin depositions make it more necessary
than ever to monitor the quantity of gold and match or tailor the profile of
the soldering process to the characteristics of the materials being processed.
- If I remember correctly, and I may not, the IPC pwb standards require a
minimum of 30 microinches of gold for Class 1 Products and a minimum of 50
microinches of gold for Class 3 Products.
Regards, Jim Moffitt, Moffitt Enterprises Consulting

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